The Highs and Lows of Grazing: Effects of Ungulates and Elevation on Grassland and Sagebrush Steppe Vegetation Composition in Yellowstone National Park

Abstract

The iconic grassland and sagebrush-steppe plant communities of the Northern Range of Yellowstone National Park have long been a research focus in conservation, largely surrounding debates on whether ungulates are overgrazing vegetation. Despite this history, and significant changes in bison and elk populations in the last two decades, relatively few studies have been conducted recently to assess the status and health of these communities. This study investigates how vegetation composition changes along the elevation gradient and under grazing conditions in YNP. I collected paired transect data inside and outside of exclosures and throughout the park to measure percent composition, height, and sagebrush volume and density. I found that along the elevation gradient, non-native species are found at higher abundances at low elevations and near park entrances, suggesting that the park is currently relatively protected from invasion as a mountain ecosystem. The exclosure results indicate that overgrazing by ungulates are reducing beta-diversity and native cover and promoting non-native cover. Differences in vegetation change were greater in bison-dominated areas than elk-dominated areas, demonstrating that bison are overgrazing more than elk. Conversely, grazing by intermediate levels of bison and elk reduced these negative impacts and even increased spatial heterogeneity. It is recommended that YNP management policies shift to recognize the impact of overgrazing by ungulate populations and increase translocations of bison to tribal lands, while invasive species policies should focus on low elevation and surrounding areas to prevent the spread of exotic plants in the park.